Device and process for diluting an aerosol

ABSTRACT

A device dilutes an aerosol without distortion due to loss of particles, especially liquid particles, on wall surfaces. An inlet pipe feeds aerosol as inlet aerosol and an outlet pipe has an inlet opening arranged at a finite distance from an outlet end of the inlet pipe. An annular space surrounds an end area of the inlet pipe. A clean gas line opens, via an outlet opening, into the annular space. A process is provided including feeding the aerosol as inlet aerosol through the inlet pipe, with particle-free clean gas fed to an end area of the inlet pipe and mixed with the fed inlet aerosol into a diluted outlet aerosol. The diluted outlet aerosol is sent to a measuring device. The mass flow of the aerosol sent to the measuring device and that of the added clean gas are maintained at a fixed ratio.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 of German Application 10 2017 011 075.1, filed Nov. 30, 2017, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention pertains to a device and to a process for diluting an aerosol.

BACKGROUND

An aerosol is a particle-carrying gas or gas mixture, especially air. Such aerosols must be tested for a great variety of reasons.

Only one particle may be present at any one time in the measured volume especially in the case of measurement methods used for testing, with which the number of particles flowing through a measured volume per unit of time is detected. This is not ensured in case of highly concentrated aerosols. If two or more particles are present simultaneously in a measured volume, this leads to coincidence errors. These are a counting error, on the one hand, because the number of particles detected is too low, since, for example, two or more particles are detected as only one particle. Moreover, there arises a size error, because a plurality of particles are viewed as only one particle, and this particle is detected as an excessively large particle.

Therefore, highly concentrated aerosols are to be diluted for this reason. A reduction of the probability of such errors or even a practically complete elimination of such errors can be achieved by the aerosol being diluted by particle-free gas. An external gas or gas mixture, especially also clean air, may be added, in principle, for this purpose. It may be disadvantageous that this gas does not have the same parameters as the carrier gas or carrier gas mixture of the aerosol, having, for example, a different moisture content and/or temperature, as a result of which the measurement results may be distorted.

It is known, for example, from the VDI Guideline VDI 3491, Sheet 15, No. 4, p. 4, that the aerosol flowing in as an inlet aerosol can be split into two aerosol flows and, on the one hand, the larger portion can be pumped through a filter, which purifies the aerosol in this branch and generates a clean gas or clean gas mixture, especially also particle-free air, and, on the other hand, a smaller portion of the aerosol can be sent through a capillary—with an internal diameter of 1 mm and with a length of 150 mm—and the two flows can again be merged downstream of the filter and the pump, on the one hand, and of the capillary, on the other hand, and the mixed flow can be fed to the sensor device.

The drawback of this procedure is, in particular, that particles, especially liquid particles, will be deposited on the wall of the capillary because of the small diameter of the capillary, as a consequence of which the composition of the aerosol leaving the capillary will change and such particles may thus also distort the measurement result. Continuous monitoring of the state of the capillary is also necessary for this reason.

SUMMARY

A basic object of the present invention is therefore to provide a device and a process, by which the undesired separation of particles, especially liquid particles, from the aerosol, which has especially a distorting effect, is avoided. Further, a more precise setting of the mixing ratio, which is necessary for inferring the particle concentration and the particle size distribution in the inlet aerosol, shall be possible.

The above object is accomplished according to the present invention with a device of the type mentioned in the introduction, which is characterized by an inlet pipe, which feeds aerosol as an inlet aerosol, by an outlet pipe with an inlet opening, which opening is arranged at a finitely spaced location from the outlet end of the inlet pipe, with an annular space surrounding at least an end area of the inlet pipe and with a clean gas line opening into the annular space via an outlet opening.

Furthermore, a process of this type is proposed for accomplishing the object mentioned for diluting aerosol, said process being characterized in that the aerosol is fed as an inlet aerosol through an inlet pipe; that particle-free clean gas is fed to an annular space surrounding an end area of the inlet pipe and is mixed with the fed aerosol to form a diluted outlet aerosol; that the diluted outlet aerosol is sent to a measuring device; and that the mass flow of the aerosol sent to the measuring device and of the added clean gas is maintained at a fixed ratio.

It is possible by means of the device according to the present invention and by means of the process, in particular, to also dilute drop aerosols to another size range of about 10 μm nearly without losses.

The term clean gas in the text designates a gas freed from liquid or solid particles as such comprised of a gas component or a gas mixture comprised of a plurality of individual gases, especially particle-free air. The annular space surrounding the inlet pipe is a space with an annular cross section. The inlet pipe for the aerosol and a measuring outlet leading to a measuring device are extremely preferably arranged aligned with a common axis. The aerosol to be measured flows along the common axis to the inlet pipe and to the measuring outlet.

Preferred embodiments of the device according to the present invention are characterized in that the distance of the inlet pipe and outlet pipe is smaller than the internal diameter of the inlet pipe or outlet pipe, especially not exceeding 6 mm, at most preferably 5 mm, and, in particular, the—radial—ring width of the annular space surrounding the end area of the inlet pipe is not greater than the internal diameter of the inlet pipe, especially corresponding to 0.5 times the internal diameter.

Provisions are made, in particular, in a variant for the internal cross section πr² of the inlet pipe and outlet pipe to be greater than 0.1 times the length L of the inlet pipe—r, L determined in mm—but be at least 12 mm², the internal diameter of the outlet pipe being greater than the external diameter of the facing end of the inlet pipe and being preferably between 1.1 times and 1.3 times the external diameter of the inlet pipe and/or the clean gas line having a mass flow controller (Mass Flow Controller—MFC) and a pump or else a controlled pump and a mass flow meter. Furthermore, a purification device may be provided in the clean gas line, an inlet of the clean gas line being in connection with a foreign (external) gas source, especially with the ambient air, and/or an inlet of the clean gas line being arranged downstream of the outlet end of the inlet pipe for the inlet aerosol; the clean gas line having a purification device for purifying the outlet aerosol entering it via its inlet opening.

Provisions are, moreover, made in variants for an optical sensor device for measuring the aerosol to be arranged between the outlet pipe and the inlet to the clean gas line; for a branch-off line, in which a pump and a mass flow controller are arranged for branching off a certain portion of the clean gas, to be connected to the clean gas line leading from the outlet pipe to the opening into the annular space; and/or for a lateral outlet opening to be provided for branching off a certain percentage of the outlet aerosol downstream of the inlet opening of the outlet pipe in the annular space and to be connected to the clean gas line. Both cascades are preferably arranged here aligned with one another, i.e., the inlet pipe and the outlet of the first stage and the inlet pipe and the (measuring) outlet of the second stage are all aligned. The measured aerosol flows to the measuring device along a common axis of the inlets and outlets.

In particular, a system for diluting an aerosol, in which two devices of the above-described type are arranged as cascades in series, may be provided according to the present invention, wherein an outlet pipe of a first, upstream device is connected to the inlet pipe of a second, downstream device.

In another embodiment, the above-mentioned process according to the present invention may be configured such that the clean gas is fed in a controlled manner from a foreign gas source, especially as purified clean air, from the surrounding area or else outlet aerosol is purified into a particle-free clean gas and this is returned for mixing with the inlet aerosol, and provisions may further be made for the inlet aerosol to be fed via an inlet pipe and for the outlet aerosol to be passed on through an outlet pipe, an internal diameter πr² of which is greater than 0.1 times the length of the inlet pipe, r and L being determined in mm, but equals at least 12 mm².

Provisions are, moreover, made in variants of the process according to the present invention for the clean gas to be sent into an annular space, whose width corresponds to 0.4 to 2 times the internal diameter of the inlet pipe; for the clean gas to be fed to the inlet aerosol in a gap between the outlet end of the inlet pipe and the inlet opening of the outlet pipe, which gap is smaller than the internal diameter of the inlet pipe or outlet pipe and preferably does not exceed 6 mm and especially 5 mm, and/or for the diluted outlet aerosol generated by mixing inlet aerosol and clean gas to enter an outlet pipe with an internal diameter greater than the internal diameter of the inlet pipe, wherein the difference of the internal diameter equals at least 1 mm to 2 mm. Finally, provisions are made in additional embodiments for the clean gas to be fed to the inlet aerosol at a volume flow ratio preset in a fixed manner for the ratio of the volume flow of the inlet aerosol to the clean gas being fed to this to be determined by a mass flow controller (MFC—Mass Flow Controller) and/or for the inlet aerosol to be fed into the mixing space through a pipe that has a smaller cross section than the mixing space and for the clean gas fed to the mixing space to be introduced into the mixing space above an outlet end of the feed pipe (as a result of which intensive mixing of inlet aerosol and clean gas is brought about).

Moreover, provisions may be made according to the present invention for excess clean gas to be removed as waste air, in which case only a fixed portion of the outlet aerosol is then branched off for purification especially downstream of the mixing space and is fed again as particle-free clean gas upstream of the mixing space and, moreover, the outlet aerosol that was not branched off is sent through an optical sensor device.

Provisions are made in another embodiment of the process for the portion of the outlet aerosol that was not branched off to be fed to another, downstream mixing space and for an additional dilution to be carried out there.

Further advantages and features of the present invention appear from the claims and from the following description, in which exemplary embodiments of the present invention are explained in detail with reference to the drawings.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic view showing a first embodiment of a device according to the present invention for diluting aerosol for carrying out the process according to the present invention;

FIG. 2 is a schematic view showing a second embodiment of the device according to the present invention; and

FIG. 3 is a schematic view showing a third embodiment of the device according to the present invention with cascaded diluting devices.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, the device 1 according to the present invention, which is shown in FIG. 1, has a mixing device with a mixing stage 2 with an inlet pipe 2.1, with an annular space 2.2 surrounding this and with an outlet pipe 2.3 with an inlet opening 2.3.1. In the exemplary embodiment shown, the outlet pipe 2.3 is connected via its outlet 2.3.2, which is called measuring outlet, directly to an optical sensor device 3, which is known per se, such as an aerosol spectrometer, for determining the particles of an aerosol in terms of the number of particles passing through a measured volume of the sensor device per unit of time and in terms of their size. The inlet pipe 2.1 and the outlet pipe 2.3 are aligned, i.e., they have a common axis A, which is also the axis of the inlet of the optical sensor device This also applies to the additional embodiment shown in FIGS. 2 and 3.

The outlet aerosol leaving the sensor device 3 via a return line 2.6 is suctioned off via a mass flow controller (MFC—Mass Flow Controller) 4 by a pump 6 with a preset throughput of, for example, 5 L/min via a purification device 5 in the form of a filter. The purification device 5 generates a clean, particle-free gas or gas mixture (hereinafter called only clean gas) from the outlet aerosol suctioned off from the sensor device 3; if the carrier gas of the inlet aerosol entering the inlet pipe 2.1 is air, clean air is generated by the filter.

This clean gas is fed again at a preset percentage, for example, 90%, i.e., at 4.5 L/minute in case of the above-mentioned suction rate of 5 L/minute, into the annular space 2.2 surrounding the end area of the inlet pipe 2.1 especially via openings 2.1.1 upstream of the outlet pipe 2.3 of the mixing state 2. To reduce the returned mass flow, a branch 2.7 with a branch pump 2.4 and with a mass flow controller 2.4 is provided, by means of which the undesired residual percentage, here equaling 5% or 0.5 L/minute, is branched off as waste air.

The inlet pipe 2.1 extends with its outlet end 2.1.2 into the annular space 2.2, which has a larger diameter than the pipe 2.1 or the outlet end 2.1.1 thereof. The returned clean gas is fed to the annular space 2.2 laterally in front of the outlet 2.1.2 of the inlet pipe 2.1.

There is a narrow distance or gap, via which the annular space 2.2 is also in connection with the inlet opening of the outlet pipe 2.3, between the outlet end 2.1.2 of the inlet pipe 2.1 and an inlet opening 2.3.1 of the outlet pipe 2.3. The gap equals 5 mm, which is a mean value, in the exemplary embodiment shown; however, it may also be 3 mm to 8 mm or may reach the value of the internal diameter of the outlet pipe. The internal diameter of the inlet pipe and outlet pipe is greater than 4 mm in the exemplary embodiment, and the diameter of the inlet of the outlet pipe 2.3 is slightly, i.e., by up to 1 mm, greater than the internal diameter of the inlet pipe 2.1. Concrete dimensions are, for example, 7 mm for the internal diameter of the inlet pipe and 8 mm for the diameter of the inlet of the outlet pipe. The wall of the outlet pipe 2.3 may be conically sloped in the inlet area, so that the diameter of the outlet pipe 2.3 corresponds in the further course to that of the inlet pipe 2.1. The internal cross sections πr² of the inlet pipe and outlet pipe are, in principle, greater than 0.1 times the length L of the inlet pipe 2.1; r and L being measured in mm as the unit of measurement, so that the pipes 2.1, 2.3 and especially the inlet pipe 2.1 do not exert any capillary effect.

The width of the annular space equals 0.5 times the internal diameter of the inlet pipe 2.1 in the exemplary embodiment shown and may be, in principle, between 0.4 times and 1.5 times the internal diameter mentioned.

Intensive mixing of the concentrated inlet aerosol entering through the pipe 2.1 with the clean gas being fed laterally via the inlet 2.2.1 is achieved due to this embodiment without a loss due to deposition of aerosol particles, especially liquid particles, and the inlet aerosol is thus transformed into a diluted outlet aerosol.

The highly concentrated inlet aerosol is fed via the inlet pipe 2.1 to the mixing stage 2 with a preset volume flow. This inlet aerosol is mixed intensively with the controlled preset volume flow of the clean gas based on the design embodiment of the mixing stage 2.

Following a start-up phase, the inlet aerosol is diluted in the measuring operation to an outlet aerosol leaving the pipe end 2.3.2 of the outlet pipe 2.3, which dilution is determined by the preset mixing ratio, here at a ratio of 1:10, which can be detected in the usual manner in the sensor device 3 during the measuring operation, i.e., especially in terms of the particle flow over time and the size of the particles. The diluted aerosol leaving the sensor device 3 is suctioned off through the filter 5 via the mass flow controller 4 and the pump 6, is purified in the process into a clean gas (mixture) and is fed again to the mixing stage 2 in the likewise described manner at the described percentage of 90% or 4.5 L/minute. In a concrete embodiment, the mass flow of the inlet aerosol equals, for example, Ve=0.5 L/minute following a start-up phase in the measuring operation of the diluted aerosol flowing through the measuring device 3, which equals Vv=5 L/minute, and of the returned, added clean gas (clean air), equaling Vr=4.5 L/minute, while the waste gas flows into the surrounding area at a rate of 0.5 L/minute.

Dilution of a concentrated aerosol is achieved by the device according to the present invention and the described process according to the present invention in terms of the particles contained in it with the same gas or gas mixture, especially air, as is present in the concentrated inlet aerosol, i.e., especially in terms of the gas composition, the gas temperature and the moisture content of the gas.

FIG. 2 shows, by contrast, a variant in which foreign gas, for example, from a pressurized gas container or even foreign air from the surrounding area, is used for the dilution rather than returned clean gas. Accordingly, the clean gas line 2.6 is not a return line, but is connected on the inlet side to a foreign gas or foreign air source, but it preferably also has a purification device in addition to a pump and to a mass flow controller in order to ensure that clean gas/clean air is indeed added to the inlet aerosol. The configuration according to FIG. 2 is otherwise the same as that according to FIG. 1. In particular, the mentioned dimensions, measuring conditions and other parameters apply as well. Reference is therefore made to the description of FIG. 1.

The procedure is likewise the same as that described in connection with the embodiment shown in FIG. 1.

The embodiment according to FIG. 3 is a cascade of mixing devices. A mixing device with a mixing stage 8 is arranged here upstream of the mixing stage 2 according to FIG. 1, or even according to FIG. 2. FIG. 2 shows, by contrast, a variant in which the embodiment of the mixing stage 8 is configured in basically the same manner as that described with reference to the embodiment shown in FIG. 1 and has an inlet pipe 8.1 with a smaller diameter compared to a surrounding annular space 8.2. An outlet pipe 8.3 is provided to an aerosol inlet pipe 2.1 corresponding to FIGS. 1 and 2. The outlet pipe 8.3 likewise has a smaller cross section than the area of the annular space 8.2 surrounding same. A lateral outlet 8.2.2 leads from this surrounding annular space 8.2 at the level of an inlet section of the pipe 8.3 to a return line 8.4, in which a purification device 8.6, a pump 8.7 and a mass flow controller 9 are arranged. The return line 8.4 opens via a lateral inlet 8.2.1 into the annular space surrounding the end of the pipe 8.1. The distance between the end of the inlet pipe 8.1 and the inlet of the outlet pipe 8.3 is greater here than in the embodiment according to FIGS. 1 and 2, because a portion of the outlet aerosol—for example, 90% in the measuring operation—is branched off from the annular space at the level of the inlet area of the outlet pipe 8.3, the distance preferably equaling up to twice the diameter of the inlet pipe 8.1. The dimensions and the dimension ratios are otherwise basically the same as in the embodiment according to FIG. 1. A mixing stage 2 corresponding to FIG. 1 then adjoins the mixing stage 8. There also could be one mixing stage according to FIG. 2, so that reference can thus be made, in principle, to the above descriptions.

A potentiated dilution of the inlet aerosol entering through the inlet pipe 8.1 can be achieved by the embodiment according to FIG. 3. If, as was described above with reference to FIGS. 1 and 2, each of the mixing devices 8, 2 brings about a dilution at a ratio of 10:1 in the embodiment according to FIG. 3, a dilution of 100:1 is brought about by the cascading according to the embodiment shown in FIG. 3.

The inlet aerosol is first diluted here in the mixing stage 8 by the suctioning of the aerosol via the outlet 8.2.2, the purification via the purification device 8.6 and via the mass flow controller, the controlled return of the particle-free clean gas thus purified via the lateral inlet 8.2.1 and by the mixing thereof with the inlet aerosol entering via the pipe 8.1. This diluted aerosol (diluted, for example, by the factor 10:1, as it is described with reference to FIG. 2) is sent into the mixing stage 2 via the intermediate pipe 8.3, 2.1. The diluted aerosol passes through the sensor device 4, is purified further via the purification device 5 into a particle-free clean gas, the gas being suctioned off via the mass flow controller. A preset quantity is branched off, as was described above with reference to FIG. 1, from the clean gas thus purified, while the preset fixed residue of the clean gas (here 90% or 4.5 L/minute) is fed via the inlet 2.1.2 to the annular space 2.2 of the mixing stage 2 and, as was said, the further dilution of 10:1 is brought about there, so that, on the whole, a dilution of the outlet aerosol flowing through the sensor device 3 at 100:1 is brought about.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. 

What is claimed is:
 1. A device for diluting an aerosol, the device comprising: an inlet pipe feeding an aerosol as an inlet aerosol; an outlet pipe with an inlet opening arranged at a finite distance from an outlet end of the inlet pipe; an annular space surrounding at least an end area of the inlet pipe; and a clean gas line opening into the annular space via an outlet opening.
 2. A device in accordance with claim 1, wherein the distance between the inlet pipe and the outlet pipe is at most equal to the internal diameter of the inlet pipe.
 3. A device in accordance with claim 2, wherein the distance between the inlet pipe and the outlet pipe is smaller than an internal diameter of the inlet pipe or the outlet pipe and does not exceed 6 mm.
 4. A device in accordance with claim 1, wherein the annular space surrounding the end area of the inlet pipe has a ring width corresponding to 0.4 to 2 times an internal diameter of the inlet pipe.
 5. A device in accordance with claim 1, wherein an internal cross section πr² of the inlet pipe is at least 8 mm².
 6. A device in accordance with claim 1, wherein an internal cross section λr² distance between the inlet pipe and the outlet pipe is greater than twice a length L of the inlet pipe, wherein r and L are determined in mm.
 7. A device in accordance with claim 1, wherein an internal diameter of the outlet pipe is greater than an external diameter of a facing end of the inlet pipe and is between 1.1 and 1.3 times the external diameter of a facing end of the inlet pipe.
 8. A device in accordance with claim 1, wherein: the clean gas line has a mass flow controller and a pump; or the clean gas line has a controlled pump and a mass flow meter.
 9. A device in accordance with claim 1, wherein the clean gas line has a purification device.
 10. A device in accordance with claim 1, wherein an inlet of the clean gas line is in connection with a gas source.
 11. A device in accordance with claim 1, wherein: an inlet of the clean gas line is arranged downstream of the outlet end of the inlet pipe for the inlet aerosol, and the clean gas line has a purification device for purifying the outlet aerosol entering the clean gas line via a clean gas line inlet opening.
 12. A device in accordance with claim 11, further comprising an optical sensor device arranged between the outlet pipe and an inlet to the clean gas line for measuring the particles of the aerosol.
 13. A device in accordance with claim 12, further comprising a branch-off line connected to the clean gas line leading from the outlet opening to an opening of the inlet pipe, the branch-off line having a pump and a mass flow controller, the branch-off line branching off a certain portion of the clean gas as waste air.
 14. A device in accordance with claim 1, wherein a lateral outlet opening, in the annular space, is provided for branching off a certain portion of the outlet aerosol downstream of the inlet opening of the outlet pipe and is connected to the clean gas line.
 15. A device in accordance with claim 1, further comprising an optical measuring device arranged downstream of the outlet pipe and through which the aerosol flows, wherein the optical measuring device comprises an aerosol spectrometer.
 16. A device in accordance with claim 15, wherein the inlet pipe and the outlet pipe leading to a measuring device are aligned with one another.
 17. A system for diluting an aerosol, the system comprising; a first device for diluting an aerosol, the first device comprising an inlet pipe feeding an aerosol as an inlet aerosol, an outlet pipe with an inlet opening arranged at a finite distance from an outlet end of the inlet pipe, an annular space surrounding at least an end area of the inlet pipe and a clean gas line opening into the annular space via an outlet opening; a second device for diluting an aerosol, the second device comprising an inlet pipe feeding an aerosol as an inlet aerosol, an outlet pipe with an inlet opening arranged at a finite distance from an outlet end of the inlet pipe, an annular space surrounding at least an end area of the inlet pipe and a clean gas line opening into the annular space via an outlet opening, wherein the first device and the second device are arranged as a cascade one after another, wherein the outlet pipe of the first device is connected to the inlet pipe of the second device.
 18. A process for diluting an aerosol, the process comprising: providing a device for diluting an aerosol, the device comprising an inlet pipe feeding an aerosol as an inlet aerosol, an annular space surrounding at least an end area of the inlet pipe and a clean gas line opening into the annular space via an outlet opening; feeding the aerosol as an inlet aerosol through the inlet pipe; feeding particle-free clean gas, via the clean gas line opening, to the annular space and mixing the fed particle-free clean gas with the fed inlet aerosol to form a diluted outlet aerosol; sending the diluted outlet aerosol to a measuring device; and maintaining a mass flow of the aerosol sent to the measuring device and that of the added clean gas at a fixed ratio.
 19. A process in accordance with claim 18, wherein the clean gas is fed in a controlled manner from an outside gas source as purified clean air, from the surrounding area.
 20. A process in accordance with claim 18, wherein outlet aerosol is purified into a particle-free clean gas and this is returned for mixing with the inlet aerosol.
 21. A process in accordance with claim 18, wherein inlet aerosol is fed via the inlet pipe having an internal cross section πr² that is at least 8 mm².
 22. A process in accordance with claim 18, wherein the inlet aerosol is fed via the inlet pipe, whose internal cross section πr² is greater than 0.1 times the length L of the inlet pipe, r and L being determined in mm.
 23. A process in accordance with claim 18, wherein the clean gas is sent into the annular space, which annular space has a width that is 0.4 to 2 times an internal diameter of the inlet pipe.
 24. A process in accordance with claim 18, wherein the clean gas is fed to the inlet aerosol in a gap between an outlet end of the inlet pipe and an inlet opening of an outer pipe, which gap is smaller than an internal diameter of the inlet pipe or the outlet pipe and does not exceed 6 mm.
 25. A process in accordance with claim 18, wherein the diluted outlet aerosol generated by mixing inlet aerosol and clean gas enters an outlet pipe with an internal diameter greater than the internal diameter of the inlet pipe, and a difference of the internal diameters is at least 1 mm to 2 mm.
 26. A process in accordance with claim 18, wherein the clean gas is fed to the inlet aerosol at a preset fixed volume flow ratio.
 27. A process in accordance with claim 18, wherein a ratio of a volume flow of the inlet aerosol to that of the clean gas fed to same is determined by a mass flow controller.
 28. A process in accordance with claim 18, wherein the inlet aerosol is fed to the mixing space through a pipe, which has a smaller cross section than the mixing space and that clean gas fed to the mixing space is introduced into the mixing space above an outlet of the feed pipe (as a result of which intensive mixing of inlet aerosol and clean gas is brought about).
 29. A process in accordance with claim 18, wherein the outlet aerosol is sent through an optical sensor device before purification.
 30. A process in accordance with claim 29, wherein excess clean gas is removed as waste air.
 31. A process in accordance with claim 18, wherein only a fixed portion of the outlet aerosol is branched off for purification downstream of the mixing space and is fed again as particle-free clean gas upstream of the mixing space.
 32. A process in accordance with claim 18, wherein the outlet aerosol that is not branched off is sent through an optical sensor device, and the optical sensor device comprises an aerosol spectrometer.
 33. A process in accordance with claim 18, wherein the aerosol to be measured flows along an axis from an inlet pipe through an outlet to a measuring device, the measuring device comprising an aerosol spectrometer.
 34. A process in accordance with claim 18, wherein a portion of the outlet aerosol that is not branched off is fed to another, downstream mixing space and an additional dilution is carried out comprising: feeding the aerosol as an inlet aerosol through another inlet pipe; feeding particle-free clean gas, via another clean gas line opening, to an annular space and mixing the fed particle-free clean gas with the fed inlet aerosol to form a diluted outlet aerosol; sending the diluted outlet aerosol to a measuring device; and maintaining a mass flow of the aerosol sent to the measuring device and that of the added clean gas at a fixed ratio. 